Automatic frequency control



Ngv. 30, 1937. N. P. cAsE AUTOMATIC FREQUENCY CONTROL Filed July 22, 1956 INVENTOR. NELSON P. CAS

ATTORNEY.

Patented Nov., 30, 1937 UNITED STATES PATENT OFFICE Hazeltne Corporation, ware a corporation of Dela- Application July 22, 1936, Serial No. 91,864

6 Claims. (Cl. Z50-20) This invention relates to modulated-carrier signal receivers, and particularly to the control of the resonant frequency of tunable oscillation circuits of such receivers.

I'he invention is especially concerned with adjusting the resonant frequency of a. tunable oscillation circuit. of a signal receiver independently of 4its main tuning adjustment, and it is especially suitable for controlling the frequency of the oscillation circuit of a superheterodyne radio receiver to maintain the intermediate-frequency carrier at a predetermined frequency for all tuning adjustments of the receiver.

In an oscillation circuit containing inductance and capacitance elements, one of which is adjustable for tuning the circuit over a desired frequency range, it is frequently desired to adjust the resonant frequency of the circuit relative to the frequency selected bythe main tuning element and independently of said element over a small fraction of the tuning range. Such an auxiliary adjustment is useful for various purposes. For example, it may be employed to facilitate the exact tuning of a superheterodyne receiver by controlling, Within narrow limits, the frequency of the oscillation circuit of the frequency changer in such receiver, independently of the main tuning means in said circuit and in response to departures of the intermediate-frequency carrier from a desired predetermined value, in order to minimize such departures and ensure optimum fidelity of reproduction; or, such adjustment may be used to shift the intermediate-frequency carrier relative to the mean resonant frequency of an intermediate-frequency selector in order to adjust the selectivity of the system.

It has heretofore been proposed to eiect such frequency adjustment by connecting across the oscillation circuit additional or auxiliary adjustable reactance means. However, in such a circuit, the reactance of the main tuning element varies over a wide range as this element is adjusted to tune the circuit over the range. The eiectiveness of the auxiliary reactance means in adjusting the resonant frequency of the circuit is ordinarily dependent on the relative magnitudes of its .impedance and those of the main reactance elements of the circuit. Hence, the effectiveness of the auxiliary reactance means tends to vary with the frequency to which the circuit is tuned. In order, therefore, to obtain equal adjustments of the resonant frequency of the circuit at all frequencies within its tuning range, different degrees of adjustment of the auxiliary reactance means are ordinarily required. For satisfactory control of the frequency of a tunable oscillation circuit, either manual or automatic, it is essential that equal adjustments of the auxiliary reactance means effect equal adjustments of the resonant frequency of the circuit for all frequencies within its tuning range. Furthermore, in the just described arrangements, the auxiliary reactance means tend substantially to limit the range of frequency over which the circuit can be tuned.

It is a primary object of the present invention, therefore, to provide a. novel improved system for adjusting the resonant frequency of .a tunable oscillation circuit independently of the main tuning means thereof.

It is a further object of the invention to provide a system of the character described, wherein equal adjustments of the frequency-adjusting means eiect substantially equal adjustments of the resonant frequency of the circuit for all frequencies within its tuning range.

It is a further object of the invention to provide a system of the character described, wherein the tuning range is substantially unaected by the auxiliary tuning means.

It is a further object of the invention to provide, in a modulated-carrier signal receiver, an

` improved system of the character described, in-

cluding means responsive to electrical conditions at a given point in the receiver, such as departures of the intermediate-frequency from a. predetermined value, for controlling the frequencyadjusting means to minimize deviations of such electrical conditions from their normal value.

In accordance with a preferred embodiment of the present invention, a modulated-carrier signal receiver includes an oscillation vcircuit: having an adjustable condenser for tuning the circuit to any selected frequency of its tuning range. Means independent of the main tuning condenser are provided for adjusting the resonant frequency of the circuit relative to the selected frequency over a. small fraction of the tuning range of the circuit. This means includes an auxiliary condenser connected in series with the main tuning condenser, and it may comprise an aligning or padding condenser for the circuit. There is also provided adjustable impedance means, comprising capacitive and resistive components and effectively connectedin parallel with the auxiliary condenser, which has such characteristics. relative to the constants of the oscillation circuit, that equal adjustments thereof eifect substantially equal adjustments oi the resonant frequency of the oscillation circuit at all frequencies within its tuning range. Moreover, by virtue of the manner in which the auxiliary adjusting means is connected in the oscillation circuit, the tuning range of the circuit is substantially unaected thereby. The frequency-adjusting impedance means may be adjusted manually or, as in the preferred embodiment illustrated, automatically in response to departures of the resonant frequency of the circuit from its normal value.

In the specific form of the invention hereinafter described, the invention is employed in connection with the oscillation circuit of a frequency changer in a superheterodyne radio receiver for.

controlling. the resonant' frequency of the oscillation circuit and, hence, the intermediate frequency developed by the frequency changer. The receiver also includes means for developing a control bias voltage which corresponds in polarity and amplitude to frequency departures of the intermediate-frequency carrier from the normal frequency thereof, resulting from oscillator drift, mistuning of the receiver, or other causes. This voltage is utilized to control a tube simulating an adjustable reactance and constituting the adjustable auxiliary impedance of the oscillation circuit, thereby7 to effect an adjustment of the frequency of this circuit which shifts the intermediate frequency in a direction to minimize departures thereof from its normal frequency.

. For a better understanding of the invention, together with other and further objects thereof,

. M reference is had to the following description taken in connection with the accompanying drawing, and its scope will be pointed out in the appended claims.

In the accompanying drawing, the single gure is a circuit diagram, partly schematic, of a superheterodyne receiver embodying the present invention.

Referring now more particularly to the drawing, there is shown schematically a superheterodyne receiver including a tunable radio-frequency amplifier I8 connected to an antenna 8 and ground 9. Connected in cascade with the radiofrequency amplifier IIJ, in the order named, are a frequency changer indicated generally at II, an intermediate-frequency amplifier I2, a detector and automatic amplification control or AVC supply I3, an audio-frequency amplifier I4,

n and a sound reproducer or loud-speaker I5. The

frequency changer I I is shown in detail and it, together with the parts of the system associated therewith which involve the present invention, Will be hereinafter further described. VIt will be understood that several parts of the system which are illustrated schematically may be conventional in their construction and operation, the details of which are well known in the art, rendering description thereof unnecessary herein.

Neglecting for the moment the particular operation of the parts of the system involved in the present invention, the system above described includes all of the features of a, conventional superheterodyne receiver. The operation of such a receiver being well understood in the art, detailed explanation thereof is deemed unnecessary. In brief, however, a desired modulated-carrier signal intercepted by the antenna is selected and amplified in the radio-frequency ampliiier I Il and converted by the frequency changer II to an intermediate-frequency signal. 'I'his signal is selected and amplified by the intermediate-frequency amplifier I2 and translated therefrom to the detector I3, wherein the audio frequencies of modulation are derived. The audio frequencies of modulation are amplified in the audio-frequency amplifier I4 and reproduced by the loudspeaker I5 in conventional manner. B iasing potentials developed by the AVC supply I3 are supplied to control the gain of one or more of the tubes of the intermediate-frequency amplier I2, and, if desired, also to control the gain of the preceding stages of the system to maintain the amplitude of the signal output of the amplifier I2 within a relatively narrow range for a Wide range of received signal amplitude.

Referring now more particularly to the portion of the system. in connection with which the present invention is employed, the frequency changer II comprises a pentagrid oscillator-modulator tube I6 having its signal input grid coupled to the radio-frequency amplifier I 0, and an oscillation circuit including an inductance 20 and a tuning condenser 2| in parallel. The oscillation circuit is coupled to the first or oscillator grid of the tube I6 by way of a suitable coupling condenser 22, and to the second grid or oscillator anode through a feed-back path comprising an inductance 24, coupled to the inductance 28, and a coupling condenser 25.

The condenser 2I is connected for unicontrol with the tuning condensers of the amplier I0 in av conventional manner, as indicated by the broken lines U, and an aligning condenser 23 is connected in series in the oscillation circuit, serving to align the oscillation circuit with the tuned circuits of the radio-frequency amplifier Ill, so that a substantially fixed intermediate frequency will be developed by the frequency changer as the tunable circuits are all simultaneously adjusted by the unicontrol means U to tune the circuits over their respective ranges.

A suitable biasing resistor I'I and by-pass condenser I'Ia are included in the cathode circuit of the tube I6, and a leak resistor I8 is connected between the oscillator grid and the cathode. Proper operating potentials are supplied to the screen from a suitable source, indicated at +Sc, and to the oscillator anode from a suitable source, indicated at +B, by way of a resistor I 9. 'Ihe frequency changer as thus far described comprises a substantially conventional circuit arrangement. the operation of which is well known'.

In accordance with the present invention, for the purpose of adjusting the frequency of the oscillation circuit 20, 2I independently of the main tuning condenser 2|, there is provided a tube 2B having an input circuit comprising in series a resistor 21, blocking condenser 28 and the inherent grid-to-cathode capacitance of the tube, which is indicated by dotted lines as a condenser 29, connected across the oscillation circuit, as shown. The blocking condenser 28 is sufficiently large as to have substantially no effect on the characteristics of the circuit at the oscillation frequencies, and the impedance of 'the resistor 2'I is large relative to that of the grid-cathode capacitance at the oscillation frequencies. The output circuit of tube 26 is connected across the series-aligning condenser 23, the cathode circuit of the tube including a suitable source of biasing voltage, indicated at +C. The tube 26A is preferably of a high impedance type, such as a pentode, operating voltages being supplied to the screen from a suitable source, indicated at +Sc, and to the anode from a source, indicated at +B, by way of a resistor 30.

For the purpose of controlling the oscillation frequency in accordance with deviations of the intermediate-frequency carrier from its normal value, due to oscillator drift, misadjustments of tuning, or other causes, so as to minimize such deviations and ensure optimum fidelity of reproduction at all times, means are provided for developing a control voltage which varies in amplitude and polarity in accordance with such deviations and for properly applying this voltage to the control grid of the tube 26 in order to effect the required frequency adjustments. These means preferably comprise a frequency discriminator and rectifier, indicated schematically at 32, coupled to the intermediate-frequency amplifier I2, and having its output circuit connected by way of a suitable filter, including a series resistor 33 and shunt condenser 34, and a resistor 3| to the control grid of the tube 26. The discriminator and rectifier 32 may be of any suitable type, various practical arrangements for this purpose being well known in the art. For example, the discriminator-rectier. may comprise a circuit arrangement, such as is described in detail in British Patent No. 443,423, accepted February 25, 1936. Preferably, however, this arrangement for providing the frequency control bias voltage will comprise a circuit arrangement similar to that disclosed in my copending application, Serial No. 72,532, filed April 3, 1936.

Referring now more particularly to the operating characteristics of the frequency control circuits, since the impedance of resistor 2'I is high compared to .that of the grid-to-cathode capacitance 29, the voltage across the capacitance 29 lags the voltage across the oscillation circuit. The circuit constants are so chosen that, at the highfrequency end of the tuning range, the voltage lag will be approximately 90 degrees. At the low-frequency end of the range the impedance of the capacitance 2S increases relative to that of the resistor 2l, so that the lag of the voltage across the capacitance 23 will be correspondingly less. The space current of the tube, being in phase with the tube input voltage, also lags the voltage across the oscillation circuit in corresponding degree. It is to be noted, however, that the voltage across the condenser 23 is in phase opposition to the voltage across the oscillation circuit, so that the space current of the tube leads the voltage across the condenser 23 and the tube simulates an impedance having capacitance and resistance components only, the value and char,- acter of which impedance varies in accordance with the frequency of the oscillation circuit, and the value of which varies also in acordance with the grid-bias voltage aiilied to the tube.

By proper adjustment of the various circuit constants, the variations of the amplitude and phase of the plate current of the tube 26 with respect to the oscillation voltage, as the oscillation circuit is tuned over its range, are controlled so that equal variations of the mutual conductance of the tube, resulting from adjustments of its grid-bias voltage, effect substantially equal adjustments of the resonant frequency of the circuit for all frequencies of its tuning range.

This relation is possible because the tube 26 is connected across the aligning condenser 23, which is connected in series with the tuning condenser 2 I. The effectiveness of this aligning condenser is dependent upon the ratio of its impedance to that of the tuning condenser 2|, which varies in a predetermined manner as the oscillation circuit is tuned over its range. Therefore,

if condenser 23, per se, were made adjustable for the purpose of' effecting the auxiliary frequency adjustment, its effectiveness would vary inversely in accordance with the frequency to which the circuit was tuned. It will be understood that the word effectiveness as here employed is intended to mean the frequency change in kilocycles of the oscillation circuit resulting from a given adjustment of the condenser 23. Obviously a given percentage frequency adjustment at a high frequency constitutes a substantially greater adjustment in kilocycles than the same percentage frequency adjustment at a low frequency. Thus, the above-mentioned variation in effectiveness of condenser 23 would be such that the percentage frequency change would be less at the higher frequencies than at the lower frequencies for a given capacitance adjustment. However, for initially aligning the oscillation circuit with the tuned radio-frequency amplifier circuits, the condenser 23 ordinarily is of such size that its percentage frequency variation would not be at the proper rate to result in equal effectiveness, that is, equal frequency adjustments in kilocycles for the same capacitance adjustment at different frequencies of the tuning range. In accordance with the present invention, therefore, there is provided an auxiliary tuning control means which, in the illustrated embodiment, comprises the tube 26 and its associated circuits as described above, the percentage frequency variation of which, with respect to the frequency to which the oscillation circuit is tuned, is correlated with the relative capacitances of condensers 23 and 2l, so that an increment of control, while effecting different percentage frequency adjustments of the oscillation circuit, effects the same frequency adjustment in kilocycles at all frequencies of the tuning range of the circuit. Furthermore, in the ar-` rangement described having the output circuit of the tube 26 connected across the relatively large aligning condenser 23 of the oscillation` circuit, the tuning range of the circuit is not limited to an appreciable extent by the effective capacitance of the auxiliary tuning means.

In considering the operation of the system as a. whole, it will be assumed that the receiver initially is exactly tuned to the desired signal frequency and the normal intermediate-frequency carrier is being developed by the frequency changer I I that no bias voltage is developed by the discriminator and rectifier 32; and that the resonant frequency of the oscillation circuit is of a proper value to give the normal intermediate frequency, taking into consideration the effect of the tube 26 with normal grid voltage applied. When,

v however, the intermediate-frequency carrier deviates from its normal frequency, due to mistuning of the oscillator with respect to the carrier wave of the desired signal, or other causes, a bias voltage is developed by the discriminator and rectifier 32 having polarity and amplitude variable in accordance with the direction and amount of the deviation. This voltage being applied to the control grid of the tube 26 effects a variation in the mutual conductance thereof, and hence a change in thev amount of apparent capacitance and resistance provided thereby effectively in parallel with the condenser 23, so as to shift the oscillation frequency in the proper direction to minimize the frequency deviation of the intermediate-frequency carrier and ensure optimum fidelity of reproduction.

Illi/'lille it is obvious that this invention is not restricted to an oscillation circuit or system employing circuit elements of any particular value, the following values were found to give satisfactory performance in a particular receiver:

In the particular system referred to, for maximum grid-bias voltage of one polarity applied to the tube 26, specifically a grid-bias of 4.5 volts, the control effect on the oscillation frequency for the various frequencies of the tuning range varied only within the range of +55 to +6 kilocycles,

-while for the maximum grid-bias voltage of the other polarity, +4.5 volts, the control effect varied only Within the range of 4.5 to -6 kilocycles.

While there has been described what is at present considered to be the preferred embodiment of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore, aimed in the appended claims to cover all such changes and modilcations as fall within the true spirit and scope ofthe invention.

What is claimed is:

1. An oscillation circuit tunable over a range of frequencies and including an adjustable condenser for tuning said circuit to a selected frequency of said range and means, independent of said tuning condenser, for adjusting the resonant frequency of said circuit relative to said selected frequency over a. small fraction of said range, comprising an auxiliary condenser connected in series with said adjustable condenser, a vacuum tube having a phase-shifting input circuit coupled to said oscillation circuit and an output circuit connected across said auxiliary condenser, said tube having such characteristics relative to the constants of said circuits with which it is associated and being so connected to said oscillation circuit that its output circuit simulates an impedance having a capacitive component and that equal variations of the mutual conductance thereof effect substantially equal adjustments of the resonant frequency of said oscillation circuit for all frequencies of said range, and means for adjusting the mutual conductance of said tube.

2. An oscillation circuit tunable over a range of frequencies and including an adjustable condenser for tuning said circuit to aselected frequency f said range and means, independent of said tuning condenser for adjusting the resonant frequency of said circuit relative to said selected frequency over a small fraction of said range, comprising an auxiliary condenser connected in series with said adjustable condenser and a vacuum tube having an input circuit including phase-sluiting means coupled to said oscillation circuit andan output circuit connected across said auxiliary condenser, said tube having such characteristics relative to the constants of said circuits with which it is associated that its output circuit simulates an impedance having a capaci- .tive component and that equal variations of the mutual conductance thereof effect substantially equal adjustments of the resonant frequency of said oscillation circuit for all frequencies of said range.

3. An oscillation circuit tunable over a range of frequencies and including an adjustable condenser for tuning said circuit to a selected frequency of said range and means, independent of said tuning condenser for adjusting the resonant frequency of said circuit relative to said selected frequency over a smallfraction of said range, comprising an auxiliary condenser connected in series with said adjustable condenser, a vacuum tube having an output circuit connected across said auxiliary condenser, and a phase-shifting circuit comprising a high impedance resistance element and the inherent gridcathode capacitance of said tube connected in series across said oscillation circuit, said capacitance having relatively low impedance at the oscillaton circuit frequencies, said tube having such characteristics relative to the constants of said circuits with whichl it is associated that its output circuit simulates an impedance having a capacitive component and that equal variations of the mutual conductance thereof effect substantially equal adjustments of the resonant frequency of said oscillation circuit for all frequencies of said range.

4. An oscillation circuit tunable over a range of frequencies and including an adjustable condenser for tuning said circuit to a selected frequency of said range and means, independent of said tuning condenser for adjusting the resonant frequency of said circuit relative to said selected frequency over a small fraction of said range, comprising an auxiliary condenser connected in series with said adjustable condenser, a vacuum tube having an output circuit connected across said auxiliary condenser, and a phaseshifting circuit comprising a resistance element and the inherent grid-cathode capacitance of said tube effectively in series across said oscillation circuit, said tube having such characteristics relative to the constants of said circuits with which it is associated and being so connected therewith that its output circuit simulates an impedance having a capacitive component and said resistance and inherent capacitance being so related that equal variations of the mutual conductance of said tube effect substantially equal adjustments of the resonant frequency of said oscillation circuit for all frequencies of said range.

5. In a Wave-signal-receiving system including a circuit tunable over a iirst range of frequencies, an oscillation circuit tunable over a second frequency range related to said rst range and having an adjustable condenser for tuning to a selected frequency thereof, said oscillation circuit being adapted for unicontrol tuning with said tunable circuit, an auxiliary aligning condenser connected in series with said tuning condenser, and means, independent of said tuning condenser .for adjusting the resonant frequency of said oscillation circuit relative to said selected frequency over a small fraction of its tuning range, comprising a vacuum tube having a phase-shifting input circuit coupled to said oscillation circuit and an output circuit connected across said aligning condenser, and control means for adjusting the gridbias voltage of said tube to eiect variations of the mutual conductance thereof in accordance with variations of the resonant frequency of said oscillation circuit, said tube having such characteristics relative to the constants of said circuits with which it is associated and being so connected to said oscillation circuit that its output circuit simulates an impedance having reactance and resistance components only and that equal variations of the mutual conductance thereof effect substantially equal adjustments of the resonant frequency of said oscillation circuit.

for all frequencies of said range.

6. In a superheterodyne wave-signal-receiving system including frequency-changing means for deriving from any selected modulated-carrier signal within a Wide range of frequencies a second modulated-carrier signal having a carrier normally of a predetermined frequency but subject to deviations Within narrow limits upon mistuning thereof, an oscillation circuit having an adjustable condenser for tuning said circuit over a range of frequencies to a selected frequency,

means, independent of said tuning condenser` for adjusting the resonant frequency of said circuit relative to said selected frequency over a small fraction of said range, comprising an auxiliary condenser connected inserles with said adjustable condenser, a vacuum tube having a phase-shifting input circuit coupled to said oscillation circuit and an output circuit connected across said auxiliary condenser, said tube having such characteristics relative to the constants of said circuits with which it is associated and being so connected to said oscillation circuit that its output circuit simulates an impedance having a capacitive component and that equal variations of the mutual conductance thereof eiect substantially equal adjustments of the resonant frequency of said oscillation circuit for all frequencies of said range, and means responsive to said frequency deviations for adjusting said mutual conductance so to vary the frequency of said oscillation circuit as substantially to reduce said deviations.

NELSON P. CASE. 

